Abstract: Marine planktonic diatoms play an important role in marine primary productivity and the carbon fixation is closely related to light intensity. In the present study, we investigated the growth rate, photosynthetic characteristics, carbonic anhydrase activity, and ribulose 1, 5-bisphosphate carboxylase/oxidase (RubisCO) activity of Phaeodactylum tricornutum and Thalassiosira weissflogii under different light intensities (high or low light). The results showed that high light intensity promote the growth of the two diatoms, which was more obvious in T. weissflogii. High light intensity attenuated the levels of chlorophyll a, c content, Fv/Fm and Yield but increased qN. High light intensity has no effect on qP. High light also enhanced the intracellular and extracellular carbonic anhydrase activities in both diatoms. Interestingly, in P. tricornutum, low light increased ribulose-1,5-bisphosphate carboxylase/oxygenase activity compared to high light; however, the opposite result was observed in T. weissflogii. Rubisco activity in T. weissflogii is higher than that in P. tricornutum in.both low and high light. These results suggest that the two diatoms are differently responded to the light intensity and that they can accordingly adjust their photosynthetic characteristics, carbonic anhydrase and RubisCO activity to accommodate the varied light intensity.